The present invention relates to end bonnets for use in a shell and tube evaporator. Shell and tube dry expansion also called direct expansion (DX) evaporator is an integral part of a refrigeration system. In a typical refrigeration system there is an evaporator that cools the process fluid at the expense of boiling the refrigerant that is at a lower saturation temperature and pressure, a compressor that compresses the boiled off refrigerant to an elevated pressure and temperature, a condenser that condenses the high pressure refrigerant to liquid phase at the expense of heating the cooling medium, and an expansion device that drops down the pressure of the condensed refrigerant back to the low side which then enters the evaporator to repeat the above cycle again. This cycle is called the reverse Rankine cycle.
A shell and tube DX evaporator generally provide a counter or cross flow arrangement for the cooling process fluid in the shell body by a refrigerant (coolant fluid) passing through the tubing within a shell body, which is frequently cylindrically shaped. This tubing provides communication between sealed opposite ends of the cylindrically shaped  configuration and defines a flow path for communication of the refrigerant from end to end of the shell structure. The tubes terminate at an end plate commonly known as tube sheets at either end of the shell and bonnet is provided at either end of this shell to define a transfer chamber for fluid communication between successive sets of tubes at each end of the shell.
Evaporators in a refrigeration cycle are generally utilized for cooling various fluids, which may be either gaseous or liquid, by refrigerant transferred through the tube arrangements. As it picks up heat from the fluid to be chilled, the coolant fluid will boil or vaporize as it flows through the tubing network extending between the bonnets. Initially during the cooling cycle, the cooling fluid is generally a liquid.
The tubes provide a tortuous path encompassing multiple passes of the coolant fluid through the shell and, as it continues to increase in temperature, the cooling fluid expands. As the cooling fluid proceeds through each successive or sequential pass, there will be a change of state for the fluid from liquid to the gaseous state. This change of state requires an expanded tube volume to accommodate the expanding cooling fluid. Therefore, subsequent cooling passes require an increased number of tubes or larger cross-sectional area tubes to transfer the initial fluid volume through the heat exchanger network of tubes. Failure to provide this increased fluid transfer volume, as the coolant fluid temperature increases until it attains the vapor state, would result in high fluid velocities in the tubes and large back pressure. In addition, problems relating to the fluid distribution result from these pressure-temperature changes. 
Abrupt increases in flow areas causes large pressure drops within the evaporators and results in decreases in pressure and thus reduction in the boiling point of the refrigerant. This characteristic indicative of a phenomenon referred to as flashing. Flashing refers to the transition from liquid to the gaseous phase due to the drop in saturation temperature. Therefore, it is desirable to limit the loss of cooling capacity due to flashing.
Bonnets of varying designs have been provided for aiding and improving fluid flow, which designs include the utilization of U-shaped return passages and inlet and outlet passages in alignment with the tubes within the housing for providing a continuous flow path through the tubes. These U-shaped passages may be provided in a flat-plate type end bonnet. However, such U-tubes are very expensive and difficult to maintain. Other prior art evaporators employ hemi spherically shaped bonnets that are subdivided by partitions or baffle plates between the flange and the contoured inner surface of the bonnet. These baffle plates thus provide transfer chambers in the bonnet between successive tube bundles of the tube network. However, the abrupt increase in flow area in the bonnets causes undesirable pressure drops.